Cleanup: Add common "ssh-login server" method

There are some "ssh-login server" methods in scenario tests, and
they will increase in the future by increasing scenario tests.
This patch adds common "ssh-login server" method to parent class
and uses it instead of each implementation.

Change-Id: Iefee90b7a79b85b67694bc32382b4c4d776bc5b2
Merge "Cleanup: Add common "ssh-login server" method"